Extended output power control of inverter

ABSTRACT

A single-thyristor series resonant inverter used to drive an induction heating coil is operated at a variable frequency over a limited ultrasonic range in known manner to obtain high output power control. The low output power range is extended by using the secondary winding of a reset transformer and a series connected auxiliary solid state device to limit the amount of recharge energy supplied to the commutating capacitor by the primary winding during non-oscillating intervals of the inverter.

United States Patent Walden 1 Oct. 15, 1974 154] EXTENDED OUTPUT POWERCONTROL 3,321,697 5/1967 Ettcr 321/45 c 01? INVE 3,349 314 /1967Giannamore... 321/45 C 1628.047 12/1971 Cronin 1 v 321/45 ER Inventor: JWalden, Schenectady, NY 3.697.716 10/1972 Kornrumpt' 11 219/1049 [73]Assignee: General Electric Company,

Schenectady, Primary ExaminerWilliam H. Shoop, Jr. Attorney, Agent, orFirmDonald R. Campbell; [22] F'led: 1973 Joseph T. Cohen; Jerome C.Squillaro [21] Appl. No.: 352,880

Related US. Application Data [57] ABSTRACT [62] Division ps 212395, Dem277 1971 A single-thyristor series resonant inverter used to drive aninduction heating coil is operated at a vari- 52 us. (:1. 321/43,219/1049, 321/45 ER able frequency Over a limited ultrasonic range in 511111.01. H02m 7/52 known manner to Obtain high Output Power Control-[58] Field of Search 307/240, 252 J, 252 M; The low Output Power rangeis extended y using the 219 1049 1077; 32 43 44 45 R, 45 C, 5 secondarywinding of a reset transformer and a series ER connected auxiliary solidstate device to limit the amount of recharge energy supplied to thecommutat- [56] References Cited ing capacitor by the primary windingduring nonoscillating intervals Of the inverter.

3,263,153 7/1966 Lawn 321/45 ER 4 Claims, 8 Drawing Figures AUXILIARYGATlNG CONTROL CIRCUIT SYNC MAIN GATING CONTROL CIRCUIT (VARIABLEREPETITION RATE) USER CONTROL PAIENTED OBI I 51574 SHEET 1 [IF 3 I I I ISTATIC POWER CONVERSION CIRCUIT SOLID STATE INVERTER (ULTRASONICFREQUENCY) RECTIFIER PATENIEUUBI 1 51m 3.842.333

SHEET 2 OF 3 MAIN GATING AUXILIARY GATING CONTROL CIRCUIT USER CONTROLCIRCUIT SYNC (VARIABLE CONTROL REPETITION RATE) gigs, 629.6

PATENIED URI 1 51974 sum 30$ 3 THYR ISTOR 33 RECHARGE CURRENT kzmmmDo 00202.0303

EXTENDED OUTPUT POWER CONTROL OF INVERTER This is a division ofapplication Ser. No. 212,295, filed Dec. 27, I971.

BACKGROUND OF THE INVENTION This invention relates to the wide rangeoutput power control of solid state inverters, and more particularly tocontrolling the output power of ultrasonic frequency induction cookingappliances employing such inverters.

Although known in principle for a number of years, the application ofinduction heating to the cooking of food was not competitive with thecommon gas range and electric range based on resistance heating untilthe development of solid state, ultrasonic frequency induction cookingappliances. These cool top appliances, as they are commonly called,comprise a static power conversion circuit typically formed by arectifier and an inverter for generating an ultrasonic voltage wave thatdrives an induction heating coil. The alternating magnetic fieldgenerated by the induction coil is coupled across a nonmetallic supportwith the bottom of the cooking utensil, which acts as a single turnsecondary winding.

A low cost, relatively simple inverter suitable for use in ultrasoniccooking equipment is a single-thyristor series resonant inverterutilizing the induction heating coil and a commutating capacitor as thebasic LC oscillator circuit. Cooking appliances incorporating aninverter of this type are described in the allowed application Ser. No.200,424 by John D. Harnden, Jr. and William P. Kornrumpf, file on Nov.19, 1971 and assigned to the same assignee as this invention now US.Pat. No. 3,781,503 dated Dec. 25, I973. The power output of the invertermust be modulated over a wide range to obtain the range of heatinglevels and cooking temperatures needed for different cookingrequirements. This is achieved by changing the ratio of oscillation timeto nonoscillation time, obtained by using a variable oper atingfrequency or by altering the series resonant frequency. Power control isalso achieved by changing the magnitude of the oscillation by using avariable input dc voltage. Wide range power control usually requires acombination of these techniques, since the feasible upper limit ofoperating frequencies is limited by eco nomic considerations. Thepresent invention is directed to another power control technique forachieving wide range control of the output power without operating ataudible frequencies.

SUMMARY OF THE INVENTION Although useful for applications other thaninduction cooking and heating, the wide range output power inverteraccording to the practice of the invention is especially suited for usein solid state induction cooking appliances. The inverter is preferablyoperated at ultra sonic frequencies and drives an induction heating coilto produce an alternating magnetic field that is coupled across asubstantially non-metallic utensil support with the cooking utensil. Thepreferred embodiment is a singlethyristor series resonant inverteroperated at a variable frequency to achieve high output power control bychanging the repetition rate of the current pulses supplied to theinduction heating coil. The inverter further includes a resettransformer having a primary winding connected to discharge energyduring non-oscillating intervals of the circuit, this energy being usedto recharge the commutating capacitor. Low output power control circuitmeans comprising the secondary winding of the reset transformer and anauxiliary solid state device selectively controls the recharge of thecommutating capacitor by preferably returning energy to the supply. Inone embodiment the auxiliary solid state device is a thyristor that isrendered conductive at different times in the delay interval betweencurrent pulses to achieve variable power control. In other embodimentsthe auxiliary device is a diode, or a diode switched to a selected tapon the secondary winding for stepped control.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a staticpower converter for supplying ultrasonic frequency power to a flatspiral induction heating coil, shown in plan view. in'a solid stateinduction cooking appliance;

FIG. 2 is a diagrammatic cross-sectional view showing the relation ofthe induction heating coil to the utensil support and cooking utensil;

FIG. 3 is a fragmentary view of an electric range with an inductioncooktop unit having a smooth utensil supporting surface;

FIG. 4 is a detailed schematic circuit diagram of a static powerconversion circuit with a one-thyristor series resonant inverterconstructed in accordance with the invention to control the output powerover a wide range;

FIGS. 5a and 5b are waveform diagrams of the induction coil current andcommutating capacitor voltage for the FIG. 4 inverter under both highoutput power and low output power conditions;

FIG. 6 is a modification of the inverter illustrated in FIG. 4 in whicha diode is substituted for the auxiliary thyristor; and

FIG. 7 is a modification of FIG. 6 to obtain stepped control at thelower end of the output power range.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The induction cooking applianceshown in FIGS. l-3

' will be described with regard to an induction surface heating unit inan electric range, but essentially the same mechanical structure andelectronic circuitry in higher and lower power versions is suitable forcommercial cooking equipment and for a portable countertop food cookingor warming appliance. The static power conversion circuit indicatedgenerally at 12 is energized by a suitable source of alternating currentpotential such as a single phase commercially available 60 Hz, or 240volt source, or by a battery source of direct current potential. Thestatic power converter I2 most commonly comprises a rectifier l3 and asolid state inverter 14 for converting the unidirectional rectifieroutput to an ultrasonic frequency voltage wave that drives the inductionheating coil 15. Induction heating coil 15 is a single layer, annular,flat spiral, air-core or ferromagnetic-core coil wound with solid flatstrip conductors or braided ribbon conductors. To generate sufficientmagnetic flux to heat the utensil to the desired level, coil 15 istightly wound with the short crosssectional dimension of the conductorsfacing upwards 253d adjacents turns separated by a flat insulating stripIn the cooking appliance (FIG. 2) induction heating coil isappropriately mounted in a horizontal position immediately below anon-metallic or substantially non-metallic support 16 typically made ofa thin sheet of glass or plastic. Support plate 16 is commonly referredto as the cooktop surface and supports the metallic cooking utensil 17to be heated. Cooking utensil 17 is an ordinary cooking pot or pan, afrying pan, or some other available metallic utensil used in foodpreparation, and can be made of magnetic or non-magnetic materials.Special cooking utensils are not required although the best and mostefficient results are obtained by optimizing the size, shape, andmaterial of the utensil. Operation of static power converter 12 toimpress an ultrasonic frequency wave on induction heating coil 15results in the generation of an alternating magnetic field. The magneticflux is coupled across the air gap through nonmetallic support 16 toutensil 17. At ultrasonic operating frequencies in the range of about1640 kHz the cooking appliance is inaudible to most people.

As shown in FIG. 3, an important feature of induction cooking equipmentis the relatively smooth and substantially unbroken utensil supportingsurface provided by support plate 16. At ultrasonic frequencies thereare no reaction forces that would cause utensil 17 to move horizontallywhen placed on the cooking surface approximately centered with respectto one of the induction surface unit positions indicated in dashedlines. Control knob 21 for each unit on the upstanding back panel oftherange turns the individual unit on and off and sets the desired heatinglevel or specific temperature to which the utensil is to be heated.Among the advantages of induction cooking are, briefly, that the surfaceof support 16 remains relatively cool; spilled foods do not burn andchar, and hence both support 16 and utensil 17 are easy to clean; andthe unobstructed utensil support is available for other food preparationand cooking tasks. The utensil is heated more uniformly than is the casewith the conventional gas range or electric resistance heating range,and the transfer of energy to utensil 17 is relatively efficient sinceheat is generated only in the utensil where it is wanted. Further, sincethere is relatively low storage of heat in the cooking utensil, theheating level or temperature to which the utensil is heated can bechanged rapidly, as from boiling to simmering to warming levels.

The preferred embodiment of power converter 12 shown in FIG. 4 uses alow cost, variable frequency one-thyristor series resonant inverter 14in which induction heating coil 15 has the dual function of providingthe commutating inductance as well as coupling power tothe utensil. Theinverter is therefore relatively simple and requires only one gatingcircuit. lnput terminals 27 and 28 are energized by a commerciallyavailable 60 Hz, 120 volt single phase source of a-e supply voltage. Thed-c power supply formed by full wave diode bridge rectifier l3 andfilter capacitor 29 supplies a relatively constant d-c input voltagebetween inverter input terminals 30 and 31. inverter 14 comprises apower or main unidirectional conducting thyristor 33 connected in seriescircuit relationship with the primary winding 35p of a reset transformer35 between input terminals 30 and 3]. A diode 34 to conduct powercurrent in the reverse direction is connected across the load terminalsof thyristor 33. A series RC circuit 33 is also usually connected acrossthe load terminals of thyristor 33 for dv/dt protection to limit therate of rcapplication of forward voltage to the device. The basic powercircuit is completed by a series connected commutating capacitor 32 andinduction heating coil 15 coupled directly across the terminals of theinverse-parallel combination of thyristor 33 and diode 34. When eitherof these power devices is conducting, capacitor 32 and induction heatingcoil 15 form a series resonant circuit for generating damped sinusoidalcurrent pulses that flow through induction heating coil 15. The primarywinding 35p of reset transformer 35 charges commutating capacitor32positively during the non-conducting intervals of the thyristordiodecombination. Each cycle of current flow is initiated by a gating pulseapplied to thyristor 33 by a variable repetition rate main gatingcontrol circuit 36. Suit able gating control circuits that can be usedare described in the previously mentioned Harnden and Kornrumpfapplication, Ser. No. 200,424 and in another allowed concurrently filedl-larnden and Kornrumpf application, Ser. No. 21 1,926, now U.S. Pat.No. 3,770,928 dated Nov. 6, I973. A user control 37, as for example, anadjustable potentiometer actuated by control knob 21 on the range backpanel (FlG. 3), sets the repetition rate of main gating control circuit36.

To extend the low output power control range of the inverter, theimprovement made by the present invention is that a reset transformer35, or two-winding reactor, is substituted for the reset inductor, orone-winding reactor, in the known induction cooking inverter (see FIG. 8of application Ser. No. 200,424. The secondary winding 35s of resettransformer 35 is connected in series circuit relationship with anauxiliary unidirectional conducting thyristor between d-c supplyterminals 30 and 31. Auxiliary thyristor 40 is poled in the oppositedirection as compared to main power thyristor 33, to conduct current inthe direction from negative d-c terminal 31 toward positive terminal 30.The RC snubber circuit 40' connected across the load terminals ofauxiliary thyristor 40 is included if needed to damp undesirable highfrequency oscillations caused by the rapid discharge of any current leftin winding 35s after commutation of auxiliary thyristor 40. Asis'indicated by the polarity dots, the two windings 35p and 35s of resettransformer 35 are connected in the circuit with opposite polarities.Accordingly, the voltage across secondary winding 35smust exceed the d-csupply voltage in order to condition auxiliary thyristor 40 for conduction by making the cathode potential lower than the anode potential.As will subsequently be explained in greater detail, an auxiliary gatingcontrol circuit 41 supplies gating pulses to auxiliary thyristor 40during the non-conducting.intervals of thyristor 33 and diode 34 at avariable time delay after auxiliary thyristor 40 becomes conditioned forconduction. The two gating control circuits 36 and 41 are synchronizedsuch that auxiliary gating pulse circuit 41 will operate at a controlledtime delay after operation of main gating control circuit 36. Suitablegating circuits that can be used for auxiliary gating control circuit 41are described in the SCR Manual, 4th edition, published by the GeneralElectric Company, Semiconductor Products Department, Electronics Park,Syracuse, New York, copyright 1967. The variable timing element inauxiliary gating circuit 41, such as an adjustable resistor, is includedin user control 37 and is arranged such that the inverter has acontinuously variable power output.

The inverter load is provided by the electrical losses in utensil 17.With respect to the utensil load, induction heating coil functions asthe primary winding of an air-core transformer. Utensil 17 functions asa single turn secondary winding with a series resistance l7rrepresenting the resistive part of the PR or eddy current losses, andhysteresis losses where applicable. The currents and voltage induced inutensil 17 when the induction surface unit is in operation aredetermined essentially by the transformer laws. The reflected utensilinductance forms a part of the total inductance of the high frequencyresonant circuit of inverter 14.

Control of the output power, and therefore the heating level ortemperature of utensil 17, is achieved at the upper end of the desiredpower range by changing the operating frequency or repetition rate ofinverter 14. The low output power control circuit actuated by auxiliarythyristor 40 is inactive at this time since no gating pulses aresupplied by auxiliary gating control circuit 41. Primary winding p ofreset transformer 35 therefore functions as a reset inductor as in theknown operation of this inverter circuit. The ultransonie operatingfrequencies of interest are between about 16 kHz and about kHz. At thelower end of this range, l6 kHz is considered to be the upper limit ornear the upper limit of human hearing for the average person. The otherend of the frequency range at 40 kHz is determined largely by economicconsiderations, in conjunction with the high frequency limitations ofavailable thyristor devices. The series resonant circuit comprisingessentially commutating capacitor 32 and induction heating coil 15 istuned to a resonant frequency that is higher than the highest desiredoperating frequency. lt will be noted that under no-load conditions withutensil 17 removed from the induction surface unit, the total inductanceincreases and there is therefore a slight change in the resonantfrequency of the series resonant circuit. Turning on the inductionsurface unit applies d-c voltage to inverter 14 and conditions the highfrequency resonant circuit for operation by charging commutatingcapacitor 32 positive as to the plate coupled to positive d-c inputterminal 30 and negative as to the plate coupled to d-c terminal 31. Theapplication of a gating pulse to main thyristor 33 by main gatingcontrol circuit 36 causes it to turn on, energizing the series resonantcircuit'essentially comprising commutating capacitor 32 and inductionheating coil 15. A damped sinusoidal current pulse flows throughinduction heating coil 15 and charges commutating capacitor 32negatively to a value exceeding the supply voltage. At this point thecurrent in the series resonant circuit reverses and a damped sinusoidalcurrent pulse of the opposite polarity flows through induction heatingcoil 15 and diode 34. Commutating capacitor 32 applies a reversebias-voltage to main thyristor 33, and turn-off is aided by the reversevoltage applied by conducting diode 34. When the current in the seriesresonant circuit again attempts to reverse, main thyristor 33 does notconduct since it has regained its forward voltage blocking capabilities,and a gating pulse is not applied to the main thyristor at this time.Because of the losses in the electrical circuit due to the heating ofutensil l7, commutating capacitor 32 at the. end of the completeconduction cycle on a steady state basis is left charged to a lowervoltage than it had at the beginning of the oscillation.

While either main thyristor 33 and diode 34 are conducting, the primarywinding 35p of reset transformer 35 is connected between the d-c supplyterminals 30 and 31, and accordingly current builds up in winding 35p.During the circuit off-time when both of power devices 33 and 34 arenon-conducting, the energy stored in primary winding 35p is dischargedand transferred primarily to commutating capacitor 32, thereby leavingcommutating capacitor 32 with a net positive charge at the end of thecircuit off-time. FIG. 5a shows the sinusoidal induction coil currentfor two complete cycles of operation separated by a time delay interval42 corresponding to the circuit off-time or energy transfer period. Thedashed line shown is the current under high output power operatingconditions. The corresponding commutating capacitor voltage under steadystate conditions with the utensil load in place is shown in dashed linesin FIG. 5b. At the end ofthe conduction cycle the magnitude of thepositive voltage on capacitor 32 is lower than the peak negativevoltage, and the action of the primary winding 35p of reset transformer35 during the interval 42 is to change the capacitor voltage almostlinearly as indicated at 43, leaving the capacitor with a net positivecharge at the end of interval 42. The function of reset winding 35p,then, is to replenish the system energy and sustain circuit oscillation.Te amount of energy transferred to capacitor 32 depends on the magnitudeof the current flowing in reset winding 35p at the beginning of theenergy transfer period and the time duration of the energy transferperiod. The energy transfer period is terminated of course when mainthyristor 33 is gated on again causing the high frequency inverter cycleto repeat. With practical component choices the circuit will transfermore energy from reset winding35p to commutating eapaeitor 32 as thetransfer period is made shorter, relative to the high frequencyoscillation period. That is. as the time interval 42 between completeconduction cycles becomes shorter. capacitor 32 is charged to a higherpositive voltage during the circuit off-time, so that the magnitude ofthe capacitor voltage oscillations and of the sinusoidal current pulsesincreases. ln summary, there are two effects that increase the power inwatts supplied to utensil 17 when the inverter operating frequency orrepetition rate is increased. There are larger and more frequentlyapplied current pulses in induction heating coil 15.

An induction surface heating unit for use in domestic electric rangesdesirably has a maximum power output of l to 1.5 kilowatts and a minimumpower output below watts. The top limit is sufficient for violent andrapid boiling while the low power output is used for warming easilyburned foods such as rice. Wide range control of the output power of theinverter of this magnitude cannot be achieved, however, solely bychanging the operating frequency within the feasible range of about l6kHz to 40 kHz. To obtain wide range power control an additionaltechnique for controlling the output power of inverter 14 is required.By way of example, low output power control as herein taught is neededto obtain output power below about 200-400 wats. The use of resettransformer 35 and auxiliary thyristor 40 extends the low output powercontrol range of the inverter and is completely compatible with variableoperating frequency control of the output power. The operation of theextended power range inverter is in many respects similar to that of thebasic inverter. As-

suming that auxiliary thyristor 40 is non-conducting, main thyristor 33is gated on to initiate one cycle of high frequency oscillation. At thecompletion of a complete cycle of conduction, recharging of commutatingcapacitor 32 by the discharge of energy stored in reset winding 35prapidly causes the capacitor voltage to become more positive than thepositive d-c supply voltage. At sometime period after this event,depending on the rate of rise of voltage across reset transformerwinding 35p and the turns ratio between windings 35p and 35s, thevoltage across secondary winding 35s exceeds the d-c supply voltage withthe polarity such that the dot end of the winding is negative. When thisoccurs, auxiliary thyristor 40 is conditioned for conduction since thecathode potential is more negative than the anode potential; Theapplication of a gating signal by auxiliary gating control circuit 41turns on auxiliary thyristor 40 and clamps secondary reset transformerwinding 35s to the d-e supply potential. This results in returningexcess energy to the supply, and forces the voltage across primary resetwinding 35p to move toward a fixed value equal to the d-c supply voltagetimes the turns ratio between windings 35p and 35s. The rate at whichthis equalization of circuit potentials occurs depends to a large extenton the leakage inductance between the two windings, the inductance ofthe induction heating coil 15, and the capacitance of capacitor 32. Thecombined inductance of the induction heating coil and the leakageinductance also fundamentally controls the rate of rise of currentthrough auxiliary thyristor 40. Auxiliary thyristor 40 is always in anonconducting condition at the start of the next conduction cycle sincethe turn-on of main thyristor 33 causes the potential across resettransformer secondary winding 35s to be positive at the dot end.Accordingly, a limit can be forced on the voltage magnitude to whichcommutating capacitor 32 is recharged, and consequently the energystored in the capacitor at the beginning of each inverter cycle andtherefore the amplitude of the current pulse and the power output of theinverter. Ordinarily the low output power control circuit is operatedwith the inverter frequency at its lower limit of 16 kHz or perhaps 20kHz to allow for harmonic content. At this operating frequency the timedelay in terval 42 between consecutive conduction cycles is relativelylong. By controlling the timing of the gating of auxiliary thyristor 40,the voltage across commutating capacitor 32 at the beginning of eachinverter cycle can be varied. Rendering conductive auxiliary thyristor40 later in the time delay interval 42 causes only a small change in thecommutating capacitor potential, while earlier gating causes a largerchange. Consequently, the low output power control range of the inverteris extended. and the low output power portion of the complete range iscontrollable.

By way of example of the operation of the low output power controlcircuit, FIG. 5a illustrates in full lines the reduced amplitude of theinduction heating coil current, assuming that the inverter repetitionrate is the same as when generating the dashed line currentcharacteristic. The corresponding full line capacitor voltage likewisehas reduced peak values and is drawn for the case in which auxiliarythyristor 40 conducts as early as possible during the energy transferperiod. Comparing the full and dashed line capacitor voltages, the netincrease in capacitor voltage during the energy transfer period isconsiderably smaller when the low output LII power control circuit is inoperation. It is believed that the capacitor voltage initially rises ata faster rate at the start of the energy transfer period due to thehigher average current level in reset transformer 35. Thereafter thereduction in capacitor voltage as energy is returned to the supply isclearly evident.

The modification of the invention shown in FIG. 6 is a low cost versionusable when the variable control feature is not needed. A diode oruncontrolled rectifier 44 is substituted in place of auxiliary thyristor40, and a series switch 45 is closed upon adjusting user control 37 orcontrol knob 21 to a low power setting. Diode 44 is forward biased andconducts when the voltage across secondary reset transformerwinding 35sexceeds the d-c supply voltage between input d-c terminals 30 and 31.There is thus a single low output power setting below the output powerobtained by operating the inverter at the lower limit of its variableoperating frequency range. The turns ratio of reset windings 35p and 35sis selected to obtain the desired low power setting. The furthermodification shown in FIG. 7 uses the diode 44 in conjunction with atapped secondary reset transformer winding 35s to implement steppedcontrol of the low power portion of the output power range. The use offour-position switch 46 connects the cathode of diode 44 to differentpoints on the secondary winding, with the result that diode 44 conductsat a later point in the time delay interval between conduction cycles asthe number of turns is reduced.

The improvement here described to extend the low output power controlrange of the inverter can also be used with the modifications ofinverter 14 shown in FIGS. 5 and 9 of the previously mentionedapplication Ser. No. 200,424. In FIG. 5 commutating capacitor 32 isconnected directly in series with coil 15 and power devices 33 and 34between the d-c supply terminals, and a single winding reset inductor isconnected across the commutating capacitor. FIG. 9 has a similararrangement with the exception that the single winding reset inductor isconnected across both coil 15 and commutating capacitor 32. Althoughunidirectional conducting thyristors or silicon controlled rectifiersare preferred in the practice of the invention. other powersemiconductors such as the triac or diac controlled in unidirectionalmode can be used. Further. it is evident that the high frequency seriesresonant circuit can be operated at a constant operating frequency, andpower control obtained solely by the use of reset transformer 35 andauxiliary thyristor 40. An inverter as herein described also has otherapplications than as a part of induction cooking appliances.

In summary, wide range control of the output power of a single-thyristorseries resonant inverter to obtain a wide range of heating levels andcooking temperatures is achieved by a simple circuit addition andwithout changing the operating characteristics of the basic inverterunder high power conditions. The low output power control range isextended by using the secondary winding of the reset transformer and aseries connected auxiliary solid state device to selectively returnenergy to the supply and thereby control the amount of recharge energytransferred to the commutating capacitor during non-oscillatingintervals of the inverter. Conseuqently, the amplitude of the powerpulses coupled to the cooking utensil is reduced. As was mentioned, thistype of wide power range inverter has utility for applications otherthan induction cooking and heating.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention.

I claim:

1. A wide range output power inverter connected between a pair ofunidirectional voltage input terminals comprising a series resonantcircuit comprising a commutating capacitor and a pair of alternatelyconducting main power devices which are operated at a variable frequencywithin a selected frequency range to generate a train of oscillatorypulses with a variable delay interval therebetween, to thereby achievehigh output power control,

a reset transformer having inductively coupled primary and secondarywindings, said primary winding being operatively connected to saidseries resonant circuit and input terminals to store energy duringconducting intervals of said main power devices and discharge energyduring the delay intervals between consecutive oscillatory pulses thatserves to recharge said commutating capacitor and sustaincircuit'oscillation, and

low output power control means for obtaining a plurality of low outputpower levels while operating within said selected frequency range bycontrolling the amount of energy discharged by said primary winding andthereby variably recharging said commutating capacitor, said low outputpower control means comprising a circuit including said resettransformer secondary winding and a series connected auxiliary thyristordevice that is selectively rendered conductive at a variable time duringsaid delay intervals.

2. A circuit according to claim 1 wherein said low output power controlmeans is connected between said unidirectional voltage input terminalsto return energy to the supply.

3. A wide range output power inverter connected be tween a pair ofunidirectional voltage input terminals comprising a series resonantcircuit comprising a commutating capacitor and a pair ofalternatelyconducting main power devices which are operated at a variable frequencywithin a selected frequency range to generate a train ofoscillatorypulses with a variable delay interval therebetween. to thereby achievehigh output power control, reset transformer having inductively coupledprimary and secondary windings, said primary winding being operativelyconnected to said series resonant circuit and input terminals to storeenergy during conducting intervals of said main power devices anddischarge energy during the delay intervals between consecutiveoscillatory pulses that serves to recharge said commutating capacitorand sustain circuit oscillation, and

low output power control means for obtaining a plurality of low outputpower levels while operating within said selected frequency range bycontrolling the amount of energy discharged by said primary winding andthereby variably recharging said commutating capacitor, said low outputpower control means comprising a circuit including said resettransformer secondary winding, a series connectable auxiliary diode, anda switch for connecting said auxiliary diode to a selected tap on saidsecondary winding, said secondary winding having a plurality of taps.

4. A circuit according to claim 3 wherein said low output power controlmeans is connected between said unidirectional voltage input terminalsto return energy to the supply.

1. A wide range output power inverter connected between a pair ofunidirectional voltage input terminals comprising a series resonantcircuit comprising a commutating capacitor and a pair of alternatelyconducting main power devices which are operated at a variable frequencywithin a selected frequency range to generate a train of oscillatorypulses with a variable delay interval therebetween, to thereby achievehigh output power control, a reset transformer having inductivelycoupled primary and secondary windings, said primary winding beingoperatively connected to said series resonant circuit and inputterminals to store energy during conducting intervals of said main powerdevices and discharge energy during the delay intervals betweenconsecutive oscillatory pulses that serves to recharge said commutatingcapacitor and sustain circuit oscillation, and low output power controlmeans for obtaining a plurality of low output power levels whileoperating within said selected frequency range by controlling the amountof energy discharged by said primary winding and thereby variablyrecharging said commutating capacitor, said low output power controlmeans comprising a circuit including said reset transformer secondarywinding and a series connected auxiliary thyristor device that isselectively rendered conductive at a variable time during said delayintervals.
 2. A circuit according to claim 1 wherein said low outputpower control means is connected between said unidirectional voltageinput terminals to return energy to the supply.
 3. A wide range outputpower inverter connected between a pair of unidirectional voltage inputterminals comprising a series resonant circuit comprising a commutatingcapacitor and a pair of alternately conducting main power devices whichare operated at a variable frequency within a selected frequency rangeto generate a train of oscillatory pulses with a variable delay intervaltherebetween, to thereby achieve high output power control, a resettransformer having inductively coupled primary and secondary windings,said primary winding being operatively connected to said series resonantcircuit and input terminals to store energy during conducting intervalsof said main power devices and discharge enerGy during the delayintervals between consecutive oscillatory pulses that serves to rechargesaid commutating capacitor and sustain circuit oscillation, and lowoutput power control means for obtaining a plurality of low output powerlevels while operating within said selected frequency range bycontrolling the amount of energy discharged by said primary winding andthereby variably recharging said commutating capacitor, said low outputpower control means comprising a circuit including said resettransformer secondary winding, a series connectable auxiliary diode, anda switch for connecting said auxiliary diode to a selected tap on saidsecondary winding, said secondary winding having a plurality of taps. 4.A circuit according to claim 3 wherein said low output power controlmeans is connected between said unidirectional voltage input terminalsto return energy to the supply.